This invention relates to an internal coating for increasing the efficiency of a centrifugal compressor. More specifically, the invention relates to a powder paint coating within the volute of a turbocharger.
There is a continuing interest and importance to improving the thermal efficiency of the internal combustion engine. One of the most efficient forms of internal combustion engines is a diesel engine with a turbocharger. It is well known that the turbocharger utilizes waste heat of the exhaust gas to drive a compressor for compressing the intake air accepted by the diesel engine.
The role of the turbocharger in improving the brake specific fuel consumption (BSFC) of a diesel engine includes not just efficiently extracting power from the exhaust gas, but also using that power to efficiently compress the intake air. One general rule of thumb is that each one percent improvement in the efficiency of the turbocharger compressor relates to a one-third percent improvement in the BSFC of the engine.
One of the problems with centrifugal compressors of turbochargers is that the centrifuging action of the compressor provides air at high velocities into a diffuser. Although the diffuser, which is also called a volute on a turbocharger, slows the air exiting from the blades of the compressor, there is still a portion of the flowpath of the diffuser in which the high velocity air scrubs over the walls of the flowpath. This scrubbing action is aerodynamic drag of the air flowing over the wall. This drag reduces the total pressure of the air and increases the total temperature of the air, and thereby reduces the efficiency of the compressor.
There are techniques for smoothing the walls of the diffuser and thereby increasing the efficiency of the compressor. One such technique is to fabricate the volute from smooth, wrought materials, but this greatly increases the cost of the diffuser. It is more common to cast the diffuser so as to make it affordable. In some applications, sand casting is a preferred technique. However, even with selection of fine grain sand, the as-cast surfaces often have a surface roughness in excess of 250 microinches. This as-cast surface can be further machined as to improve its smoothness, although this additional machining operation necessarily increases the cost of the diffuser. One such technique is extrude honing.
What is needed is a technique for increasing the efficiency of centrifugal compressors in a cost-effective manner. The present invention does this in a novel and unobvious way.